Method for checking gas analysis devices

ABSTRACT

A method for checking gas analysis devices in respect of their sensitivity and their response and regeneration behaviour with respect to time, wherein a reproducible quantity of the measurement component is produced at a predetermined production rate, for a period of time which is guided entirely or at a constant proportion as a concentration pulse to the measurement element of the gas analysis device. For the production of the measurement component a chemical or electro-chemical reaction or even a thermal reaction is used. The time interval can then be conveniently controlled. A decisive advantage with toxic measurement components is that the gas production takes place in all cases for only a short time during the test procedure.

The invention relates to a method and apparatus for checking gasanalysis devices with respect to their sensitivity and their responseand regeneration behaviour with respect to time.

A regular function check on automatic gas analysis devices is necessarybecause of their high susceptibility to faults. This is frequentlycarried out manually. Both in automatic measurement grids as arefrequently constructed today for emission monitoring and also forprocess monitoring in the chemical industry and particularly in ambientair monitoring in endangered work places, the requirements for atrouble-free and regular function check of the analysis device must bemet. The same requirements must be set in respect of reliability andoperator convenience as are already realised in computer controlledremote control systems and as are necessary for their meaningful use. Insuch a closed fully automatic chain ##STR1## TODAY THE MONITORING UNITIS THE WEAK LINK. The function check of the analysis device is generallyeffected by means of a calibration gas, which is given off by acalibration point continuously by the dilution or permeation principle.These methods are very complex, and require relatively large stocks ofthe measurement component, which for reasons of toxicity is frequentlyundesirable and in addition they are very slow. To obtain relevantinformation, the operation of the analysis device must be interruptedfor an objectionably long time. During this time the analysis devicecannot fulfil its actual function. This operation interruption cannot betolerated if the device is for example used for control or alarmpurposes. In the case of the diffusion measurement head in increasinguse for ambient air monitoring based on electro-chemical measurementcells or semi-conductor measurement cells, automatic function checkingwith the hitherto conventional methods is in principle impossible.

Function checking is intended to mean that the gas analysis device canbe examined simultaneously at least in respect of its sensitivity andits response and regeneration behaviour with respect to time. The methodis intended also to be suitable for gas analysis devices in which themeasurement gas is guided past the measurement element (through-flowmeasurement element) in a continuous flow, and also for devices in whichthe measurement component passes by diffusion to the measurement element(diffusion measurement element). Since toxic gases are frequently used,for safety reasons it is necessary that in principle large stocks of themeasurement gas components are not present in the containers (e.g. gascartridges). In addition it is important that the function check can betripped automatically by the computer via the remote control system andevaluated.

According to the invention there is provided a method for checking gasanalysis devices with respect to their sensitivity and their responseand regeneration behaviour with respect to time, wherein in a timeinterval Δt_(E) at a predetermined production rate m_(E) (t), areproducible quantity ##EQU1## of the measurement component is produced,either a constant proportion or all of which is guided as aconcentration pulse to the measurement element of the gas analysisdevice.

A concentration pulse is taken in analogy with the pulse in mechanics tomean a quantity of the measurement component which within time Δt_(M)acts on the measurement element, which is small in relation to thefollow-up time τ₉₅ (95% value) of the gas analysis device. In practicethe range used is Δt_(M) ≦0.1τ₉₅. The concentration pulse arises eitherdirectly through the mechanism chosen for the gas production or isproduced by special auxiliary agents from a prepared quantity.

According to a particular embodiment of the invention, the quantityM_(E) is produced by a chemical reaction which is spontaneously inducedand then terminated after a predetermined time interval Δt_(E).

Another embodiment is characterised in that the quantity M_(E) isreleased by the brief opening of a reservoir filled with the measurementcomponent. Both methods can also be combined.

Particularly suitable for this method are reactions in which thereagents are brought into contact with one another momentarily.

However, for the method according to the invention an electro-chemicalreaction with an electrolytic decomposition is preferably used. Thebeginning and end of the reaction can then be controlled particularlyeasily by closing or opening an electrical contact. The quantityproduced of the measurement component can then be calculated from thequantity of electrical energy supplied. Alternatively the chemicalreaction used to produce the quantity M_(E) can be controlled by athermal reaction. A further alternative is that the chemical reaction iscontrolled by an electrical discharge. In addition, photo-chemicalreactions are suitable for the method according to the invention. Bymeans of momentary radiation, e.g. with ultra-violet light, a series ofreactions can be conveniently controlled.

Various methods can be used to produce the concentration pulse. Methodswhich have proved themselves include mechanical compression, theaddition of propellant, or a thermal expansion in which the gas quantityproduced is compressed through an aperture between the place of gasproduction and the measurement element. In the case of gas analysisdevices based on through flow measurement cells, the quantity producedis usefully injected into the inlet pipe of the measurement cell. In thecase of devices based on diffusion measurement heads, the concentrationimpulse is advantageously directed straight at the measurement head.

Refinements of the invention and methods for the production of specialmeasurement gas components are described in the sub-claims.

The method according to the invention permits a very rapid functioncheck with high information value. The simultaneous checking ofsensitivity and time behaviour of the analysis device is particularlyimportant when the time behaviour of the analysis device goes into theevaluation of the measurement result. This is the case:

1. in the correlation of emission measurements,

2. in process regulation,

3. in alarm systems in gas endangered rooms.

Point 1

As rapidly altering concentrations of different components in the aircan only be correlated when the time behaviour of the different gastrace analysis units is known, the running up and regeneration time ofthe devices must be continuously checked.

Point 2

Frequently, control algorithms cannot be used or found, because thevarying time behaviour of the analysis device cannot be monitored. Tooptimise the control algorithm a regular checking of the time behaviourof the analysis devices is necessary.

Point 3

The time and regeneration behaviour of the analysis device (warningdevice) is here of essential significance.

The regular monitoring of the time behaviour is therefore indispensable.

A further important advantage of the monitoring of gas analysis devicesaccording to the invention is that the test gases in general can beproduced with a small and manageable apparatus. In particular using anelectro-chemical, photo-chemical or thermal decomposition reaction,reproducible quantities can simply be produced, with a proportionalrelationship generally existing between the electric current or heatenergy applied and the gas quantity produced. Using these methods it ispossible always to produce the same quantity discontinuously. The energyrequirement is low. For the production of quantities in the trace rangeit lies in the order of magnitude of a few Watt-seconds. This permitsthe construction of a small manageable apparatus for the functiontesting. In contrast to the methods according to the state of the art,no pressurised container with the test gas is required. Thus in the caseof toxic test gases, there is no danger to the personnel. In addition,the majority of the methods described hereinafter have proved themselvesalso for the continuous quantitative production of gas traces.

In the accompanying drawings:

FIG. 1 shows the electro-chemical production of a quantity of gas withsubsequent mechanical compression for diffusion measurement heads;

FIG. 2 shows the incorporation of a H₂ S generator into a through-flowmeasurement cell;

FIG. 3 shows the dependence of the H₂ S production on the water vapourcontent of the carrier gas;

FIG. 4 shows the dependence of the H₂ S production on the electrolysiscurrent;

FIG. 5 shows a test gas generator for the production of a phosgeneconcentration pulse; and

FIG. 6 shows a spark chamber for the production of NO_(X) concentrationpulses.

The Function Monitoring of Analysis Devices By Means of ConcentrationPulses (Measurement Principle)

To test the gas analysis device, a small quantity M_(E) of themeasurement component is produced at the production rate m_(E) (t) inthe time Δt_(E) : ##EQU2## This quantity is enclosed in a volume V_(E)and as later described is pushed out by thermal expansion or mechanicalcompression through an aperture or capillary. The measurement componentissues from the capillary or aperture at the through-flow rate m_(K) (t)during the time Δt_(K) : ##EQU3## M_(K) ≦M_(E) takes into account thefact that optionally only a fraction of the gas quantity M_(E) producedis driven out through the capillary or aperture. This concentrationpulse M_(K) is then supplied to the measurement element of the gasanalysis device. In this arrangement the temporal distribution m_(K) (t)changes according to the supply mechanism to m_(M) (t), so that thequantity M_(M) ≦M_(K) is supplied to the measurement element accordingto the function m_(M) (t) during the time Δt_(M). ##EQU4##

M_(M) ≦M_(K) takes into account as in the above case the fact thatoptionally only a fraction of the quantity expelled from the capillaryor aperture arrives at the measurement element. The measurement devicethen reacts to the incoming temporal distribution m_(M) (t) of themeasurement component over the time Δt_(A) with its particular responsefunction m_(A) (t). ##EQU5##

The response is attributed to a quantity of 1 component only which ispermitted if a calibrated measuring device is used.

The transformation of the concentration pulse

    m.sub.K (t).sup.T m.sub.M (t)

is in practice adapted in each case to the measurement element and themeasurement task. In the case of a through flow measurement cell forexample, the measurement component m_(K) (t) is injected into an inletpipe and supplied to the measurement element by means of a flow. As aresult the concentration pulse is spread. The initially almostrectangular distribution is "smeared." ##STR2## In the case of adiffusion measurement cell, the measurement component m_(K) (t) isdirected straight at the cell. The transformation is thereforeidentical. ##STR3## In practice the function monitoring of gas analysisdevices is effected according to the above described method in such away that the indication m_(A) (t) produced by one concentration pulse isin each case compared with the preceding indication. As long as eachconcentration pulse produces the same indication m_(A) (t), the deviceis in order. However if the indication m_(A) (t) changes with the _(i)th function check to m_(A) ^(i) (t), then a function fault of themeasurement device can be concluded from this change. The followingcases can be simply interpreted:

1. The evaluation of the maximum indication m_(Amax). With very shortconcentration pulses the sensitivity can be directly checked. If forexample it is ascertained that

    m.sub.Amax.sup.i <m.sub.Amax.sup.v

(m_(Amax) ^(v) comparative value) then it can be concluded that thesensitivity of the device has dropped.

2. The evaluation of the integral: if for example it is ascertained that

    M.sub.A.sup.i =∫m.sub.A.sup.i (t)dt<M.sub.A.sup.v

then the sensitivity of the device has dropped.

3. The evaluation of the differential. The time behaviour of the devicecan be checked by means of the first differential of the response curveat the turning point ##EQU6## (a) If on the front flank of themeasurement signal it is ascertained that ##EQU7## then the running uptime of the device has increased.

(b) If on the rear flank of the measurement signal it is ascertainedthat ##EQU8## then the regeneration time of the device has increased.

Criteria of the type specified can be checked and interpreted by thecomputer. The method specified thus completes the fully automatic chain,by the computer tripping the function monitoring via the remote controlsystem and initiating action according to the results. Only in the caseof an extreme fault is manual servicing by dialogue required.

Embodiments for the production of gas concentration pulses are describedin the following.

1. Production of Gas Traces by a Chemical Reaction Which is Controlledby the External Pressure p as a State Variable

In systems composed of several reactants the state depends apart fromthe pressure and temperature on the numbers of moles of all thereactants present. During the course of the reaction the numbers ofmoles change constantly until the end state is achieved. According tothe law of mass action, K is designated as a thermodynamic equilibriumconstant of the reaction: ##EQU9## in which σ represents the equivalencenumbers and a the equilibrium activity.

If a reactant D is present as a gas, then its concentration in theliquid corresponds according to the Henry-Dalton law to the gas pressureover the mixture. In a closed system, the pressure increases markedlywhen the gas being produced is only slightly soluble in the liquidphase. With increasing pressure the solubility of the gas increasesuntil its concentration in the solution has reached the equilibriumconcentration corresponding to the law of mass action. At that point nofurther chemical reaction can take place. The pressure p as a statevariable determines the release of the quantity of gas. In order totransform this quantity into a concentration pulse, the auxiliary agentsdescribed further below are used.

In an open system the gas is able to escape, so that its concentrationin the liquid remains low. As a result of the stoichiometric laws, thechanges in the numbers of moles of the reactants are determined by thechange of the remaining reactants. On reaching equilibrium, only smallquantities of the starting reaction partners A and B are finallypresent, i.e. the reaction is practically completely concluded. By theappropriate selection of the quantities of the reactants A and B, thequantity of the test gas produced can be varied within wide limits.

The two methods mentioned above can be combined. By continuous ordiscontinuous regulation of the gas removal from an otherwise closedsystem, the closed system can be converted into an open system, so thatin it a chemical reaction takes place as required and as a result adesired quantity of gas is released.

In practice, the release of the quantity of gas can be achieved by thebrief opening of a valve, as a result of which the equilibrium state ofthe system is temporarily disturbed. The stoichiometric reaction torestore the equilibrium then corresponds to the gas quantity M_(E) givenoff. This method is described with reference to the known Scheelereaction. In this reaction chlorine is produced by the oxidation of HClwith MnO₂. In this process chlorine and momentarily MnO are formed.Manganese oxide (MnO) then reacts with HCl to form MnCl₂.

    ______________________________________                                        2 HCl + MnO.sub.2 ⃡ Cl.sub.2 + H.sub.2 O + MnO                    MnO + 2HCl ⃡ MnCl.sub.2 + H.sub.2 O                               ______________________________________                                        4 HCl + MnO.sub.2 ⃡ Cl.sub.2 ↑ + MnCl.sub.2 + 2 H.sub.2     O.                                                                            ______________________________________                                    

If this reaction is conducted in an open system, then a quantity isproduced according to the stoichiometric conversion of the reactants.However if the reaction takes place in a closed container, the pressurerises considerably as a result of Cl₂ production and so does thesolubility of the chlorine in the liquid phase until the reaction isstopped in the state of equilibrium. If the closed container is providedwith a valve, then the reaction can be restarted by the brief opening ofthe valve.

2. The Production of the Quantity of Gas by a Chemical Reaction, inWhich Two Reactants are Momentarily Brought into Contact with OneAnother

A defined quantity of chlorine can, for example, be produced by brieflyguiding HCl vapours over an oxidation mass of manganese IV-oxide andpotassium bisulphate. The manganese IV-oxide and potassium bisulphateare in this arrangement usefully applied onto ceramic granulate as asupport. The reaction takes place as follows:

    ______________________________________                                        2 HCl + MnO.sub.2 ⃡ Cl.sub.2 ↑ + H.sub.2 O + MnO            MnO + 2 KHSO.sub.4 ⃡ MnSO.sub.4 + H.sub.2 O                       ______________________________________                                        2 HCl+MnO.sub.2 +2 KHSO.sub.4 ⃡Cl.sub.2 ↑+MnSO.sub.4        +K.sub.2 SO.sub.4 +2H.sub.2 O.                                                ______________________________________                                    

In this way any desired quantity of chlorine can be produced during aspecific time Δt_(E). Δt_(E) is simply determined by the passing-overtime.

3. The Production of a Concentration Pulse by Means of ElectrolyticDecomposition

Quantities of gas for the method according to the invention can beproduced particularly conveniently by electrolytic decomposition. FIG. 1shows schematically a suitable apparatus. The measurement element 2 ofthe gas analysis device is located on the test gas generator 1. Themeasurement element 2 is here an electro-chemical diffusion measurementcell as for example described in German Offenlegungsschrift No.2,436,261. FIG. 1 indicates only the electrolyte 3 of the cell with therelevant electrodes. The three phase boundary 4 responsible for themeasurement effect is connected via a diaphragm 5 to the test gasgenerator 1. The diaphragm 5 works simultaneously as an aperture for thegas input.

The main component of the test gas generator 1 is the electrolyte 6 withthe electrodes 7 and 8. By applying a voltage to the electrodes 7 and 8,an electrolytic decomposition is produced, whereby the desired quantityof gas is produced. To produce a concentration pulse, a cylinder 9 isarranged above the electrolyte cell. 6. A piston 10 is connected to theupper sealing plate 11 of the test gas generator. By depressing thesealing plate 11, the piston 10 also moves downwards so that thequantity of gas located above the electrolyte 6 is compressed to producea concentration pulse. Through the axial bore 12 located in the piston,the concentration pulse is guided directly to the measurement element 2.In practice, a piston stroke of 2to 5 mm with a cross-sectional area ofbetween 0.5 and 1 cm² is sufficient.

The electrodes 7 and 8 are connected via a switch 13 to a currentsource. The switch 13 is coupled mechanically to the covering plate 11in such a way that the current circuit is only closed on depressing thecovering plate 11. The electrolytic decomposition therefore begins atthe same moment when the measurement element 2 is placed on the test gasgenerator 1 and the covering plate 11 is pushed down. A pressure spring14 ensures that the covering plate 11 returns automatically to itsstarting position when the plate 11 is released.

In one embodiment which has particularly proved itself, the electrolyte6 is a hygroscopic substance. In this way, by absorbing water from theambient air, drying out is avoided, so that the conductivity necessaryfor electrolysis is always present. An improvement of the life and timestability can be achieved by arranging beneath the electrolyte6--separated by a cavity 15--an electrolyte reservoir 16 containing thesame electrolyte substance. The reservoir 16 is subject to a diffusioninteraction with the electrolyte 6 via the short cavity 15(approximately 2 mm), so that the water content of the generatorelectrolyte 6 remains practically constant as a result of theself-adjusting diffusion equilibrium. The reservoir function 16 comesinto effect both with excessively dry and excessively humid climates. Inthe latter case, the electrolyte 6 in communication with the ambient airgives off its excess water content by diffusion to the reservoir 16.

A suitable hygroscopic substance is for example potassium bisulphate.Particular compositions based on this substance are described furtherbelow.

In particular in the production of traces of hydrogen sulphide andchlorine it has been found that the production rate is greatly dependenton the relative humidity of the air. In the arrangement according toFIG. 1 the electrolyte 6 can be regarded as the sediment of a saturatedsolution, in which the proportion of the liquid phase determines theelectro-chemical property. A determining factor is the conductivity inthe region of the three phase boundary between electrolyte, electrodeand gas chamber. Optimum conditions in the liquid phase are present whenthe partial vapour pressure of the water in the gas phase is equal tothe pressure in the liquid phase. This can be achieved by the gas phasebeing adjusted to the desired water vapour pressure by a receivercontaining the saturated solution of the electrolyte or by theelectrolyte being surrounded by a reservoir of the same solution.

Instead of mechanical compression according to FIG. 1 to produce theconcentration pulse, a propellent can also be used. This variant isparticularly suitable for through-flow measurement cells, in which thegas to be examined is guided in a stationary flow past the measurementelement. For checking the function of gas analysis devices based ondiffusion measurement cells, the above described mechanical compressionor the thermal production of the concentration pulse as described belowshould be used.

4. The Electrolytic Production of Hydrogen Sulphide

FIG. 1 shows the function testing of a diffusion measurement cell. FIGS.2 to 4 illustrate the production of a quantity of hydrogen sulphide forchecking the function of a through flow measurement cell. The H₂ Sgenerator is here incorporated in a feed pipe to the measurement cell(see FIG. 2). The electrolyte 6 in this case consists of 50% by weightpotassium bisulphate, 30% by weight sulphur and 20% by weight water.This mixture is stirred into a paste, heated to approximately 95° C. andpoured into a plastics ring 17. The mixture hardens on cooling. Oneither side of the electrolyte electrodes 7, 8 are provided. Theyconsist of carbon felt or carbon fabric, to provide the necessary gaspermeability. On the application of a voltage to the electrodes 7, 8,hydrogen is produced at the cathode, which reacts with the sulphurcontained in the electrolyte to form hydrogen sulphide:

    2H.sup.+ +2e→2H.sup.o

    2H.sup.o +S→H.sub.2 S.

the electrolysis cell 6, 7, 8 is arranged in a cylindrical housing 18and is held on either side by sealing caps 19 and 20.

The quantity of hydrogen sulphide produced is conveyed by means of apropellent to the measurement cell. A suitable propellent is an inertgas to which the measurement cell does not respond. The propellent flowsthrough the bore 21 in the sealing cap 19 to the anode 8 and past theelectrolyte 6 to the cathode 7. There the hydrogen sulphide issuing fromthe cathode is delivered with the propellent through the bore 22 in thesealing cap 20 out of the electrolysis cell. The electrode terminals areguided outwards through the sealing caps 19 and 20. The sealing caps 19and 20 are each inserted with an O-ring 23 in gas-tight manner into theplastics housing 18.

An electrolysis cell constructed in this way was examined in respect oflong term behaviour in the presence of water vapour in the propellent.It was found that:

1. The maximum hydrogen sulphide production takes place with low watervapour content in the propellent (see FIG. 3). In this case the H₂ Squantity produced corresponds almost to the theoretical value.

2. With permanent operation there was no exhaustion of H₂ S production.

The curve in FIG. 3 shows the H₂ S production as a function of the watervapour content of the propellant. It can be divided into two areas:

1. With increasing water vapour content up to 0.75 g/m³ the H₂ Sdevelopment increases simultaneously.

2. With a water vapour content from 0.75 g/m³ there is a decreasing H₂ Sproduction.

In general with increasing water vapour content the proportion of theliquid phase of the solid electrolyte becomes greater. As a result thereis the danger of flooding at the critical three phase boundary betweenelectrode 7 and electrolyte 6. Since the H₂ S partial pressure isdependent on the concentration of the dissolved H₂ S, the concentrationof the H₂ S in the gas phase initially decreases. On the other hand bythe reduction of the liquid phase, an increase of the internalresistance of the electrolyte is effected. The voltage increase thusoccuring leads as a result of the consequent competition reaction to alower yield of H₂ S. At a humidity of 0.75 g H₂ O/m³ and 20° C. the H₂ Syield is greatest. The concentration then measured is 28. 10⁻³ ppm (seeFIG. 3). The theoretically produced concentration is calculated as##EQU10## in which I=electric current,

m=mass reacted,

A=electro-chemical equivalent,

t=time,

Q=gas through-flow,

c=gas concentration and

n=number of electrons reacted (=2 for H₂ S).

PARTICULAR EXAMPLE

For I=4 μA there results theoretically at Q=50 l/h a concentration##EQU11##

the efficiency of the yield is: ##EQU12##

From the dependency of the H₂ S production on the electrolysis current(FIG. 4) there results a running up behaviour of the concentration atcurrents below 2 μA. This behaviour occurs even more strongly at lowerhumidity. At higher current the dependency is virtually linear. Thisindicates parallel reactions in the H₂ S production. Their proportionincreases with lower water vapour content.

Instead of the elctrolyte mixture H₂ O+S+KHSO₄ for the production of H₂S, phosphoric acid was also used. In this case the cathode consisted ofa sintered carbon plate with 40% by weight of sulphur. Upon theelectrolytic decomposition, hydrogen sulphide was then produced at thecathode in addition to hydrogen. The phosphoric acid electrolyte wasapplied to a glass frit, i.e. the electrolyte here is present in amatrix of a solid inert body.

5. The Electrolytic Production of Chlorine

As the electrolyte a mixture of potassium bisulphate KHSO₄ and potassiumchloride KCl is used. This mixture is again highly hygroscopic andconstitutes a medium strength acid of very low vapour pressure. As theelectrodes again porous carbon electrodes are used. The electrolyte isproduced by mixing 60% by weight KHSO₄, 20% by weight of KCl and 20% byweight of H₂ O. This mixture is stirred to a paste, heated toapproximately 95° C. and poured into a plastics ring. This mixturehardens by cooling.

When a voltage is applied to the electrodes, chlorine gas is produced atthe anodes:

    2Cl.sup.- →Cl.sub.2 ␣+2e.

Systematic investigation regarding the influence of water vapour in thepropellent were also conducted with this electrode-electrolyte-system inan arrangement according to FIG. 2. It was found that the maximum Cl₂production is at a relatively high water vapour content in thepropellent (approximately 85%). In permanent operation over 2 months noexhaustion of the Cl₂ production was detected.

With increasing humidity, the proportion of the liquid phase of thesolid electrolyte becomes greater, which causes a dilution of thedissolved chlorine. On the other hand, by the reduction of the liquidphase, the contact area between electrode and electrolyte becomessmaller. This effects a current density increase at the electrodes. Thevoltage increase thus occurring leads to a lower chlorine yield becauseof competition reaction. This latter process essentially determines theCl₂ yield.

6. The Electrolytic Production of Nitrogen Dioxide

As the electrolyte a mixture is used of 60% by weight potassiumbisulphate KHSO₄, 20% by weight ammonium nitrate NH₄ NO₃ and 20% byweight of water. This mixture is stirred to a paste, heated toapproximately 95° C. and poured into a plastics ring. The mixturehardens by cooling. The design and method of functioning of theelectrolysis cell corresponds to the apparatus described under 5 for theproduction of chlorine. When a voltage is applied to the electrodes,hydrogen is produced at the cathode and nitrogen dioxide at the anode.

7. The Electrolytic Production of Atomic Mercury Vapour

As the electrolyte a mixture is used of 75% potassium bisulphate KHSO₄,5% by weight mercury-(I)-sulphate Hg₂ SO₄ and 20% by weight of water.This mixture is stirred to a paste, heated to approximately 95° C. andpoured into a plastics ring. This mixture hardens on cooling. The designand method of functioning of the electrolysis cell corresponds to theapparatus for the production of hydrogen sulphide, or chlorine. As theelectrode material carbon felt is used. When a voltage is applied to theelectrodes, vapour-form atomic mercury is produced at the cathode.

    Hg.sup.+ +e→Hg.sup.o.

The electrical power supply for the electrolytic gas productionaccording to the Examples 4 to 7 is in all cases constructed in the sameway. Either an electronic time circuit ensures that during a specifictime Δt_(E) a constant current flows or a capacitor is dischargedthrough the electrolysis cell. The capacitor has for example acapacitance of 2,500 μF. The charge voltage is approximately 9 volts.The quantity of gas produced is in this case directly proportional tothe charge stored in the capacitor.

8. The Production of a Phosgene Concentration Pulse

The function checking of phosgene analysers causes problems because ofthe high toxicity of phosgene. It is therefore an important requirementthat only such small quantities of phosgene are produced during such ashort time as are necessary for the testing of the analysis device. Forthis purpose an apparatus shown in FIG. 5 has proved itself. Thephosgene generator according to FIG. 5 consists of a Teflon housing 24with a reaction reservoir 25 and a thermal reaction chamber 26. From thereaction reservoir 25 a porous ceramic pipe 27 leads into the reactionchamber 26. The reaction reservoir 25 consists of hexachloroacetone,which together with polymethyl methacrylate forms a gel. Thehexachloroacetone vapours rising through the ceramic pipe 27 are reactedin the reaction chamber 26 thermally to form phosgene. The reactiontakes place on a platinum wire 28 which can be heated electrically withapproximately 5 watts. Temperatures of approximately 500° C. areproduced. The reaction takes place according to the equation: ##STR4##

As a result of simultaneous heating in the reaction there takes place anexpansion of the quantity of phosgene produced, so that a concentrationpulse is generated. This concentration pulse escapes through an aperture29 at the upper end of the reaction chamber 26. The aperture 29 has adiameter of approximately 0.3 mm. At a short interval above the aperture29 the diffusion measurement cell 2 to be tested is arranged. It hasalready been described with reference to FIG. 1.

Carbon tetrachloride can be used as the reaction liquid instead ofhexachloroacetone. The reaction then takes place according to thefollowing equations:

    CCl.sub.4 +O.sub.2 (air oxygen)→2COCl.sub.2 +Cl.sub.2

    CCl.sub.4 +H.sub.2 O (air humidity)→COCl.sub.2 +2HCl.

To heat the heating wire 28 for a short time it is connected via aswitch to a capacitor. The capacitor has for example a capacitance of4,400 μF and is charged to a voltage of 9 volts. By means of this simplecircuit current flows through the heating wire only for the short timeinterval Δt_(E). In the discharge of this capacitor there is produced inthe arrangement according to FIG. 5 for example a phosgene pulse of 20ppm for a duration Δt_(E) =0.1 s. In principle the apparatus accordingto FIG. 5 can also be conveniently used as a calibration gas generatorfor stationary tests. For this purpose the capacitor circuit is merelyreplaced by a circuit which delivers a constant current.

9. The Production of Nitrogen Dioxide by Means of an ElectricalDischarge

In an electrical spark discharge in air, NO and NO₂ gas traces areproduced.

    N.sub.2 +O.sub.2 +42.1 kcal⃡2NO

    2no+o.sub.2 ⃡2no.sub.2 +28 kcal.

The apparatus for producing NO_(x) traces by means of electric sparks isshown in FIG. 6. The spark section 30 is incorporated in a glasstube-cross piece 31. It consists of two platinum wires of 0.5 mmdiameter at a distance of 2 mm. To be able to adjust the distancebetter, the electrodes are arranged movably (movable Teflon guides 32).To test a through-flow measurement cell the spark section isincorporated in its feed pipe. A supply device (not shown) ensures aconstant air flow through the spark chamber and through flow measurementcell. By additionally incorporating an oxidation filter it is possiblefor the NO proportion to be quantitatively oxidised to NO₂.

The spark section is operated with a high voltage source (e.g. atransistor ignition system of Messrs. Bosch). With each spark there isproduced a nitrogen dioxide concentration pulse which is transported tothe measurement element.

In the course of miniaturization the high voltage source was replaced bya piezo-quartz. By pressure on the piezo-quartz a voltage pulse can beproduced which leads to a single spark discharge. With each spark thereis produced a concentration pulse of nitrogen dioxide which istransported to the measurement element. To test diffusion measurementcells the spark section is brought into the immediate vicinity of themeasurement element.

The apparatus according to FIG. 6 is also suitable for permanentoperation, i.e. as a calibration gas generator. In this case the highvoltage source is controlled by a frequency transmitter so that sparksare produced periodically. At a spark frequency of 1.2 Hz and anelectrode distance of 3 mm, for example a permanent NO₂ concentration isproduced in the order of magnitude of 0.4 ppm.

What we claim is:
 1. A method for checking gas analysis devices withrespect to their known sensitivity and their known response andregeneration behavior with respect to time, wherein in a time intervalΔt_(E) at a predetermined production rate m_(E) (t), a reproduciblequantity ##EQU13## of the measurement component is produced and at leasta constant proportion of which is conveyed to the measurement sensor ofthe gas analysis device, the temporal distribution of the quantity istransformed into a concentration pulse which is applied to the sensorwithin a period Δt_(M), which is an order of magnitude shorter than theknown response time of the analysis device and the response of the gasanalysis device to the concentration pulse received at the sensor isevaluated.
 2. A method as claimed in claim 1, wherein the quantity M_(E)is released by the brief opening of a reservoir filled with themeasurement component.
 3. A method as claimed in claim 1, wherein thequantity M_(E) is produced by a chemical reaction, which isspontaneously initiated and then terminated after the time intervalΔt_(E).
 4. A method as claimed in claim 3, wherein the reactants for thechemical reaction are momentarily brought into contact with one another.5. A method as claimed in claim 3, wherein the chemical reaction iscontrolled by an electrolytic decomposition.
 6. A method as claimed inclaim 3, wherein the chemical reaction is controlled by a thermalreaction.
 7. A method as claimed in claim 3, wherein the chemicalreaction is controlled by an electrical discharge.
 8. A method asclaimed in claim 3, wherein the chemical reaction is controlled by thesupply of electromagnetic energy.
 9. A method as claimed in claim 1wherein the quantity of gas produced is compressed by one of mechanicalcompression, the addition of a propellent or thermal expansion throughan aperture between the place of gas production and the measurementelement.
 10. A method as claimed in claim 1 wherein the quantity of gasis injected into the inlet pipe of the gas analysis device.
 11. A methodas claimed in claim 1 wherein the concentration pulse is directedstraight at the measurement head of the gas analysis device.